Atomic absorption and fluorescence spectroscopy

ABSTRACT

In analysing a sample by atomic absorption or atomic fluorescence spectroscopy, the cloud of free atoms which interact with incident radiation is produced by heating a graphite body, disposed below the interaction region, on which the sample has been deposited. The body is heated in an inert atmosphere by passing electric current through it. Compact forms of apparatus which may replace the burner system of a conventional spectrometer utilise a horizontally disposed graphite rod around which flows a stream of inert gas.

United States Patent 191 West [541 ATOMIC ABSORPTION AND FLUORESCENCESPECTROSCOPY [75] Inventor: Thomas Summers West, Croydon,

England [73] Assignee: National Research Development Corporation,London, England 22 Filed: Jan. 13,1970 21 Appl. No.1 2,532

[30] Foreign Application Priority Data J Jan. 16, 1969 Great Britain2590/69 52 us. Cl. 356/85 51 Int. Cl. .1 G01j 3/30 58 Field of Search356/85, 87; 250/435 R,

[56] References Cited OTHER PUBLICATIONS LVov; Spectrochimica Acta; Vo1.17'; August,- 1961; pp. 761 to 770.

Massmann; Spectrochimica Acta; Vol. 238; April,

1451 Jan. 7, 1 975 1.968; pp- 215 to 226. Woodriff et al.; AppliedOptics; Vol. 7, No. 7; July, 1968; Pp. 1337 to 1339.

Brandenberger; Chimia; Vol. 22, No. 12; December,

Primary E.taminer-James W; Lawrence Assistant ExaminerB. C. AndersonAttorney, Agent, or Firm -Cushman. Darby and Cushman ABSTRACT Inanalysing a sample by atomicabsorption or atomic fluorescencespectroscopy, the cloud of free atoms which interact with incidentradiation is produced by heating'a graphite body, disposed below theinteraction region, on which the samplehas been deposited.

The body is heated in an inert atmosphere by passing electric currentthrough it. Compact forms of apparatus which may'rcplace the burnersystem of a conventional spectrometer utilise a horizontally disposedgraphite rod around which flows a stream of inert gas 5 Claims, 4Drawing Figures o "In", finnnnnnf PATENTEB JAN 7 I975 SHEET 1 BF 2ATOMIC ABSORPTION AND FLUORESCENCE SPECTROSCOPY In the techniques ofatomic absorption and atomic fluorescence spectroscopy, as used for-theanalytical determination of various elements, a cloud of free atoms inthe vapour phase is produced from a sample under investigation andradiation of an appropriate wavelength to interact with the atoms iscaused to impinge on thiscloud, measurements being made either of theproportion of the incident radiation which is transmitted through thecloud or of the intensity of the fluorescent radiation emittedfrom thecloud as a result of excitationby the incident radiation.

Various means are known whereby the cloud of free atoms may be produced,but the one most commonly used, particularly in commercially availableapparatus, involves spraying of a solution of the sample into a flame.This method is, however, very ine fficientand is subject to the usualrisks involved in the use of flames.

It is an object of the present invention to provide a relatively simplemeans, which avoids the disadvantages involvedjin the'use of a flame,for producing free atoms from a sample under investigation by atomicabsorption or atomic fluorescence spectroscopy.

According to the invention, in a method of atomic absorption or atomicfluorescence spectroscopy'the cloud of free atoms which interacts .withincident radiation is produced from a sample under investigation byheating a graphite body on which the sample has been deposited, so as toevaporate the sample, the body being disposed wholly below the region inwhich the interaction occurs ,and being heated by the passage through itof an electric current while it is surrounded by an inert atmosphere.

The graphite body may suitably be in the form of a slender rod disposedwith its axis horizontal. The body may be disposed in a chamber throughwhich a stream of inert gas is caused to flow, the chamber being pro-FIG. 3 is a side elevation, partly cut away to show internal details ofan alternative apparatus suitable for use in either atomic absorption oratomic fluorescence spectroscopy, and 7 FIG. 4 is a plan view of part ofthe apparatus shown in FIG. 3.

Referring to FIG. 1, the apparatus shown therein comprises a chamber ofborosilicate glass made in two parts, a base 1 and a cover 2, theseparts mating by means of a cone joint 3 which in use is sealed withgrease to make the chamber gas-tight. The base 1 has sealed through ittwo tungsten rods 4 and 5 to the ends of which inside the chamber arerespectively secured two stainless steel terminals 6 and 7. In theterminals 6 and 7 are releasably clamped the ends of a rod 8 ofspectrographically pure graphite, such as is obtainable from MorganiteLtd.; the rod8 may suitably have a diameter of l to 2-millimetres, thelength of the rod 8 between the terminals 6 and 7 being about 2centimetres. The main part ofthe cover 2 is of cylindrical form, havinga length of about centimetres and a diameter of about 6 centimetres, andthis part has sealed to it two cover 2. also has sealed to it an inlettube 17, normally 7 rod 8. In use the parts 1 and 2 are fitted togetherso that vided with optical windows forthe passage of radiation into andout of the chamber; alternatively the body may I be arranged so that itis normally exposed to the ambient atmospherega curtain of inert gasbeing projected around the body when it is heated so as to shield itfrom the ambient atmosphere.

Apparatus for use in a method according to the invention may comprise aslender rod of graphite the ends of which are releasably secured toterminals via which an electric current may be passed through the rod,and means enabling an inert atmosphere to be maintained around the rod,the arrangement being such that with the apparatus disposed so that theaxis of the rod ishorizontal a sample to be investigated may bedeposited on the rod, radiation from an external source may be directedhorizontally into a region just above the rod, and radiation emerginghorizontally from said region may be detected by an external detector. i

Thus in performing the invention use may be made of arrangements such asare illustrated in the accompanying drawings, in which:

FIG. 1 is a sectional side elevation of an apparatus suitable for use inatomic absorption spectroscopy,

FIG. 2 is an underneath plan view of part of an apparatus generallysimilar to that shown in FIG. 1, but modified for use in atomicfluorescence spectroscopy,

the axes of the side arms 9 and'lO are parallel to the axis of the rod 8and lie slightly above the terminals 6 and 7.

The apparatus shown in FIG. 1 is intended for use in a conventionalatomic absorption spectrometer in place of the usual bumer-nebulizersystem, being disposed inthe instrument between the detection system anda suitable light source so that a beam of radiation (indicated by theline 19 in FIG. 1) from the light source passes through the chamber, viathe windows 1 l and 12, to the detection system; the beam 19 is directedparallel to the axes of the side arms 9 and 10 and passes just above therod 8, which thus occupies approximately the same position as would theprimary cone of the flame produced by a conventional burner. The rod 8is arranged to be heated by passing through it, via the rods 4 and 5, anelectric current which may suitablybe derived from a mains supply via astepdown, transformer having a relatively low output voltage (say 6 8volts). The temperature to which the rod 8 is heated will depend uponthe particular element which is to be analysed; it can of course bedetermined by choice of the diameter of the rod 8 and the magnitude ofthe current, and can be readily monitored by means of an opticalpyrometer. Typically the temperature may be in the range 2000 2500C, inwhich case a current of the order of amperes will be required when usinga rod 8 of the dimensions indicated above.

In operation, a sample to be investigated is deposited on the rod 8, inthe form of an aqueous solution, by means of a micrometer syringeintroduced through the tube 17, the rod 8 having previously been heatedby passing current through it and then allowed to cool to a temperatureof the order of 100C. After a period sufficient to allow the water toevaporate, the rod 8 is again heatedby passing current throughit forabout five seconds, so as to volatilise thesample and produce a cloud offree' atoms which rises into the path of the beam'l9 above the rod 8,and a measurement is made of the intensity of the light transmittedthrough the chamber; this is of course a comparative measurement withreference to the intensity of the light transmitted through the chamberwhen the rod 8 is unheated.

In order to avoid oxidation of the rod 8 it is necessary to maintain aninert atmosphere in the chamber during operation, preferably byarranging for a stable flow of an inert gasto occur through the chamber.This ensures that the water which evaporates initially from the rod 8 isswept out of the chamber before atoinisation of the sample occurs, andthe sensitivity of the instrument can beenhanced by choosing the flowrate so as to increase the rate of diffusion of atomic vapour away fromthe heated rod 8 without at the same time unduly-increasing the rate atwhich the vapour is swept out of the chamber. In practice. satisfactoryresults have been achieved by using a flow of argon or nitrogen, at apressure slightly above atmospheric," with aflow rate of a few litresper. minute.

'moved by heating-it for a sufficient periodwith the inert'gas flowing.A rod 8 having the dimensions indicated above can normally be used formany individual analyses before needing to be replacedawhen necessary,replacementof the rod 8 can be readily effected.

The base 1 with the rod 8 mounted on it may be used in conjunction witha modified form of cover to constioptical bar, the upper end of thepillar 27 being hollow and thespace within it connecting with a supplytube 28 for inert gas and a circular aperture 29 formed in the baseplate 23. Within'the aperture 29 is disposed a series of concentricrings of metal foil 30 which are alternately crimped and plain so as toprovide a multiplicity of inlet passages for producing a laminar flow ofinert gas into the chamber. The dome 22 has formed at its upper end agas outlet tube 31 provided with a tap 32.

Extending through the base plate 23 are two stainless steel terminalposts 33 and 34 respectively connectedto external leads 35 and 36respectively, the terminal post33 being in contact with the base plate23 and the terminal post 34 being electrically insulated from thebase-plate 23 by means of insulating bushes such as 37. The terminalposts 33 and -34 are hollow, their upper ends being closed, ,and inuseof the apparatus are artube 42. 1

Connected between the terminal posts 33 and 34 is a graphite rod.43similar to the rod 8, each end of the rod 43 being clamped between theupper end of one of the terminal posts 33 and 34and a correspondingdetachable stainless steel plate 44 or 45. The rod 43 has a small notch46 formed in it half-way along its length,

r the axis of the rod 43.

tute an apparatus generally similar to that shown -in FIG. 1 butsuitable for use in atomic fluorescence spectroscopy. As shown in FIG.2,the cover 2 in this case is generally similar to the cover2 but hasside arms 9 and 10' disposed with their axes at right angles in place ofthe side arms 9 and l0, the axes of the side arms 9', and 10 being.disposed at a level corresponding to that of the axes of the side arms9 and 10. In this case, in use a beam of radiation (indicated by theline 20 in FIG. 2) enters the chamber through the side arm 9' so as tointeract with a cloud of free atoms produced above the rod 8, and thedetection system is arranged to respond to fluorescent radiation emittedby the cloud and emerging from the chamber through the side arm 10" (asindicated by the line 21 in FIG. 2).

Referring now to FIGS. 3 and 4, the apparatus shown therein is basicallysimilar to, but rather more compact than, the apparatus shown in FIG. 1and is intended for use in a similar manner. In this case, the apparatuscomprises a chamber incorporating a borosilicate glass dome 22 havingdimensions similar to those of the main part of the cover 2, and a metalbase plate 23, the dome 22 having sealed to its mouth a metal flange 24which is held against the base plate 23 by means of a removable metalclamping ring 25, the' joint between the flange 24 and the base plate 23being sealed by means of a rubber ring 26 disposed ina circular groovein the base plate 23. The base plate 23 is secured to a metal pillar 27which may be held in a clamp mounted on an As indicated above, theapparatus shown in FIGS. 3 and 4 is used inasimilar manner to that shownin FIG. 1, theradiation passing through'the windows 47 and 48 in thecase of atomicabsorption spectroscopy and pass- I ing through one ofthesewindows and the window 49 in the case of atomic fluorescencespectroscopy. Since in this case no provision is made for theintroduction of a samplewhile the dome 22 is in position, the dome 22must, 'of course, be removed to allow a sample to be deposited on therod 43 and then replaced before a measurement is made. The provision ofthe forced cooling of the terminal posts 33 and 34 has two advantages.Firstly, it enables more reproducible results to be ob- 'tained, bysubstantially eliminating any tendency for the contact resistance at theends of the rod 43 to vary as the rod 43'is heated; secondly it shortensthe intervals at which it is possible to make investigations on a seriesof samples, say to a length of about 1 minute.

In certain. cases, particularly when using very intense radiationsources, it may be found that the presence of the dome 22 gives rise todifficulties due to unwanted reflections. In such cases it is possibleto operate the apparatus with the dome.22 removed, provided thatprecautions are taken to ensure that the rod 43 is shielded from theambient atmosphere by a curtain of inert gas when it is heated. This mayreadily be achieved by providing an extension of the gas inlet almost upto the level of the rod 43. For example, the foil FIGS. 3 and 4)occupying most of the space between the terminal posts 33 and 34, theupper end of this box being filled with foil arranged similarly to thefoil 30.

lt will be appreciated that the apparatuses described above arerelatively simple, compact and safe in operation. By using them in themanner described, it is possi ble to achieve very high atomisationefficiencies while avoiding any complicationsdue to background effectssuch as are encountered with flames. It is thus possible to detect verysmall amounts of individual elements; for example, in the case of atomicabsorption spectroscopy it has been found possible to determine silverand magnesium at levels of the order of gram, while in the case ofatomic fluorescence spectroscopy it has been found possible to determinesilver at a level of the order of 10 gram, magnesium at a level of theorder of 10 gram, and cadmium at a level of about 3 X 10 gram.

1 claim:

1. Apparatus for use in spectroscopy comprising a slender rod ofgraphite, a pair of terminals, means releasably securing the ends ofsaid rod to said terminals in a horizontal position for depositing asample on said rod, means for supplying an electric current to saidterminals which may be passed through the rod, means for providing aninert atmosphere around the rod, means for directing radiation from anexternal source horiz0ntally into a region just above the rod, and meansfor detecting the radiation emerging horizontally from said region.

2. Apparatus according to claim 1, in which means are provided forcooling the terminals by the circulation of fluid.

3. Apparatus according to claim 1, in which the rod is disposed in achamber having an inlet and an outlet via which a stream of inert gasmay be caused to flow through the chamber, the chamber being providedwith optical windows respectively disposed to permit the passage intothe chamber of the radiation from an external source and the passage outof the chamber of the radiation emerging from said region.

4. Apparatus according to claim 3, in which the chamber comprises a basesupporting the terminals and a detachable cover sealed to the base.

5. Apparatus according to claim 1, in which the rod is arranged so thatit is normally exposed to the ambient atmosphere, means being providedfor projecting a curtain of inert gas around the rod so as to shield itfrom

1. Apparatus for use in spectroscopy comprising a slender rod ofgraphite, a pair of terminals, means releasably securing the ends ofsaid rod to said terminals in a horizontal position for depositing asample on said rod, means for supplying an electric current to saidterminals which may be passed through the rod, means for providing aninert atmosphere around the rod, means for directing radiation from anexternal source horizontally into a region just above the rod, and meansfor detecting the radiation emerging horizontally from said region. 2.Apparatus according to claim 1, in which means are provided for coolingthe terminals by the circulation of fluid.
 3. Apparatus according toclaim 1, in which the rod is disposed in a chamber having an inlet andan outlet via which a stream of inert gas may be caused to flow throughthe chamber, the chamber being provided with optical windowsrespectively disposed to permit the passage into the chamber of theradiation from an external source and the passage out of the chamber ofthe radiation emerging from said region.
 4. Apparatus according to claim3, in which the chamber comprises a base supporting the terminals and adetachable cover sealed to the base.
 5. Apparatus according to claim 1,in which the rod is arranged so that it is normally exposed to theambient atmosphere, means being provided for projecting a curtain ofinert gas around the rod so as to shield it from the ambient atmosphere.